The present invention relates to ignition compositions, and more particularly to ignition compositions for inflator gas generators utilized in vehicle occupant restraint systems.
A steel canister is commonly utilized as the inflator pressure vessel for an automobile occupant restraint system because of the relatively high strength of steel at elevated temperatures. However, emphasis on vehicle weight reduction has renewed interest in the use of aluminum in place of steel in such pressure vessels.
One test that vehicle occupant restraint inflator systems must pass is exposure to fire whereupon the gas generating material of the inflator is expected to ignite and burn, but the inflator pressure vessel must not rupture or throw fragments. Steel pressure vessels pass this test relatively easily because steel retains most of its strength at ambient temperatures well above the temperature at which the gas generant autoignites. Aluminum, however, loses strength rapidly with increasing temperature and may not be able to withstand the combination of high ambient temperature and high internal temperature and pressure generated upon ignition of the gas generant. If, however, the gas generant of the inflator can be made to autoignite at relatively low temperatures, for example, 150.degree. C. to 210.degree. C. (302.degree. F. to 410.degree. F.), the inflator canisters can be made of aluminum.
Providing autoignition compositions for use in aluminum pressure vessels has heretofore been problematic. U.S. Pat. No. 4,561,675 granted to Adams et al., which discloses the use of Dupont 3031 single base smokeless powder as an autoignition gas generant, is exemplary of an unreliable known autoignition composition. While such smokeless powder autoignites at approximately the desired temperature of 177.degree. C. (.apprxeq.350.degree. F.), it is largely composed of nitrocellulose. One of ordinary skill in the propellant field will appreciate that nitrocellulose is not stable for long periods at high ambient temperatures and is thus unreliable as an autoignition compound. Moreover, smokeless powder autoignites by a different mechanism than the compositions of the instant invention.
In addition, commonly assigned U.S. Pat. No. 5,084,118 to Poole describes other autoignition compositions, which comprise 5-aminotetrazole, potassium or sodium chlorate, and 2,4-dinitrophenylhydrazine. While the compositions disclosed in U.S. Pat. No. 5,084,118 autoignite and cause ignition of the gas generant when heated to approximately 177.degree. C. (.apprxeq.350.degree. F.), the compositions have not proven to be fully satisfactory due to oversensitivity to shock or impact, while also being difficult and hazardous to manufacture. Difficulty in manufacture is further compounded because the Department of Transportation (DOT) classifies these compositions as Class A or Class 1.1 explosives and, as such, regulations require special facilities for manufacturing and storage.